The present invention pertains to a dye-based inkjet ink set and more particularly to an ink set with at least six differently colored inks. The ink set includes at least a cyan, magenta, yellow, red, blue and green ink.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a substrate, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor. Inkjet printers offer low cost, high quality printing and have become a popular alternative to other types of printers.
Color characteristics of dye-containing inks play an important role in the quality of the printed ink-jet image. Perceived color quality can be characterized using any one of several color-space systems such as CIELAB (CIE No. 15.2, Colorimetry, 2nd. Ed., Commission Internationale de l'Eclarage, Vienna, Austria, 1986) or Munsell (Munsell Book of Color, Munsell Color Co., Baltimore, Md., 1929), as is well known in the art. With respect to Munsell color space, a given color is defined using three terms, namely Hue, Value and Chroma. With respect to CIELAB color space, a color is defined using three terms L*, a* and b*. In this system L* defines the lightness of the color and it ranges from 0 (black) to 100 (white). The terms a* and b* together define the hue, where a* ranges from a negative number (green) to a positive number (red) and b* ranges from a negative number (blue) to a positive number (yellow). Additional terms such as hue angle (h°) and chroma (C*) can also be used to describe a given color instead of a* and b* wherein
                              h          o                =                              tan                          -              1                                ⁡                      (                                          b                *                                            a                *                                      )                                              Equation        ⁢                                  ⁢        1                                          C          *                =                                            a                              *                2                                      +                          b                              *                2                                                                        Equation        ⁢                                  ⁢        2            
To achieve full color images, ink jet printers typically employ a cyan (“C”), magenta (“M”) and yellow (“Y”) ink. These colors are known as subtractively-mixing primaries, as light is subtracted as it passes through the colorant. When these colors are mixed in pairs they form red, green and blue (so-called “secondaries”), and when all are mixed together they form black. Thus these three-color inks can be used to print the entire range of hues with good chromaticity and/or color saturation.
The range of colors that a given ink set can print is called the color gamut, and can be described in the CIELAB system by the volume in the L*a*b* color space that encompasses a palette of representative colors that can be printed by the ink set on a given substrate. While a CMY ink set can produce all possible color hues, the color gamut does not encompass all the colors that can be perceived by the human eye. Thus there is still a desire to improve the printed image quality and extend the range of colors that can be printed.
One approach to improved color is the development of new cyan, magenta and yellow dyes that give higher chroma on paper while still maintaining or improving other desired characteristics such as light-fastness and water-fastness. This approach is taken, for example, in U.S. Pat. No. 6,053,969 and U.S. Pat. No. 6,673,140.
Another approach is to expand the ink set used for printing to include additional inks that improve image quality or color gamut. There are three ways in which expanded ink sets have been employed.
One way is by including inks of the same hue but lower concentration (so-called “lights”). Light colors improve image quality by producing smoother tonal gradations and light colors in an image. For light colors, the “light” or “photo” inks, which typically have 10%-20% of the dye content of the full-strength colors, are used. For intermediate strength colors, “light” and full-strength inks are mixed. For the darkest colors only full-strength ink drops are used. Thus a smooth gradation of color tones can be obtained.
A further advantage of using “lights” is that it allows the dye concentration of the full-strength inks to be increased further without having the problem of dark sparsely spaced dots on the image. The higher concentration inks produce higher chroma secondary colors when mixed on high quality photo papers, and they also produce a darker CMY process black. These two effects serve to enhance the color gamut significantly.
U.S. Pat. No. 5,833,743 discloses how up to three different concentrations of each of the primary hues can be used to greatly increase the color gamut obtainable from a CMY ink set. Typically “light” inks are only used for cyan and magenta as yellow is light enough so that any graininess is not objectionable.
U.S. Pat. No. 6,509,916 discloses a printing method where a light ink of a different hue is added to the three-color set in order to improve improved gradation reproduction and expand the color gamut.
A second way in which an expanded ink set is employed is by including both a dye-based black (“K”) ink and a pigmented black ink (along with CMY dyes). Most commercial ink jet printers use a pigmented black ink as this gives high optical density and sharp text on plain paper; however, the pigmented black has lower gloss than the dyes and looks flat when used with dyes on photographic papers. Thus printers use a combination of CMY to achieve black when printing photographs to maintain high gloss images. The optical density of such a combination block, however, is lower than using a true black ink and also requires three times as much ink to be used, so photo printers use a special high optical density black dye ink in addition to CMY to improve the images. Previously mentioned U.S. Pat. No. 6,673,140 discloses specific black dye inks to be used to improve color gamut of CMY ink sets.
A third way an expanded ink set is employed is by further including more inks with hues other than CMY. In some cases this additional ink may be what is known as a spot color, where the image to be printed contains an abundance of a certain color and so that color is specifically added to the ink set. This spot color is not necessarily outside the existing color gamut of the CMY ink set. In the case where the desired color is inside the gamut, adding the ink still provides the advantage of using less ink since a single ink is used rather than a mixture, and it also may provide improved light color tones, again by avoiding using a mixture of other colors. As another example the digital image to be printed may contain a certain color that is difficult to accurately print with the existing ink set and that color can be selected as an additional ink for the ink set. U.S. Pat. No. 6,588,879 discloses a method for selecting inks for textile printing that addresses these two cases. The ink set disclosed as being preferred contains 12 inks.
U.S. Pat. No. 6,336,721 discloses a method for multi-color printing using a first set of CMY (and optionally K) inks, and a second set containing a specific chemical formulae for a yellow dye, and/or an orange dye, and/or a red dye and/or a blue dye. The disclosure shows how inks made from mixtures of these dyes can expand the color gamut of the existing CMYK ink set.
U.S. Pat. No. 6,508,549 similarly discusses an ink set containing a specific magenta dye which is part of an ink set containing a yellow dye ink, a cyan dye ink and an orange and/or green and/or violet ink.
U.S. Pat. No. 5,988,791 discloses that when an additional color, in this case blue, is added to the ink set for textile printing, mixtures of light cyan and magenta can be used to print the lighter blue shades.
All of the above-identified publications are incorporated by reference herein for all purposes as if fully set forth.
The art described above attempts to improve image quality by making piecemeal additions to the basic CMY ink set to address specific issues; however, none of the solutions in themselves provides both a range of colors able to reproduce photographs and computer monitor images as well as reproduce light color tones. In addition many of the solutions assume that it is a simple matter to add colors to the ink set. This may be the case in many conventional (non-inkjet) printing processes; however, in inkjet printing the number of ink slots available is both fixed by the printer design and also limited due to the scanning nature of much ink-jet printing which limits the width and weight of the printhead.
There is, consequently, a need for an ink set that integrates (a) a systematic method of expanding the color gamut, with (b) a method of printing light tones, highlights and grayscales using (c) a limited number of ink channels. It is an object of this invention to provide an ink set that address these issues.